Title Page

Contents

Abstract 11

Chapter 1. Introduction 14

1.1. Introduction to two-dimensional transition dichalcogenides (2D-TMDs) 14

1.1.1. Defect engineering in 2D-TMDs 14

1.1.2. Various methods for defect analysis in 2D-TMDs 15

1.1.3. Novel perspective on defect analysis in 2D-TMDs via scanning transmission electron microscopy (STEM) 18

1.2. Defect characterization using STEM 20

1.2.1. Basic principle of STEM 20

1.2.2. Imaging modes in STEM 22

1.2.3. qSTEM based STEM image simulation 25

1.2.4. Defect characterization via STEM-HAADF images & simulations 27

References 37

Chapter 2. Atomic-scale vacancy healing mechanism in monolayer MoS₂ via chemical treatment 40

2.1. Introduction 40

2.2. Experimental 42

2.2.1. Sample preparation 42

2.2.2. Chemical treatment 42

2.2.3. Optical characterization 43

2.2.4. STEM imaging & simulation 44

2.2.5. DFT calculation 45

2.3. Result and discussion 46

2.3.1. PL & Raman characterization 46

2.3.2. Quantitative analysis of sulfur and vacancy concentrations in pristine & treated 1L-MoS₂ 51

2.3.3. DFT calculation 62

2.4. Conclusion 65

References 67

Chapter 3. Lattice engineering of monolayer ReS₂ for modulating In-plane anisotropic property via Li doping 74

3.1. Introduction 74

3.2. Experimental 76

3.2.1. Sample preparation 77

3.2.2. XPS & optic analysis 77

3.2.3. STEM image-based strain analysis 78

3.2.4. DFT calculation 79

3.3. Result and discussion 80

3.3.1. Electron doping process via Li treatment 80

3.3.2. Nanodomain evolution in 1T'-ReS₂ via Li treatment 82

3.3.3. Quantitative lattice distortion analysis of treated ReS₂ 87

3.3.4. Polarization-resolved optical characterization of 1L-ReS₂ before & after Li treatment 95

3.4. Conclusion 106

References 107

Chapter 4. Assessing the substitutional doping of transition metals exhibiting a marginal Z difference in 2D-TMDs 113

4.1. Introduction 113

4.2. Experimental 115

4.2.1. STEM image-based strain analysis 115

4.2.2. STEM image simulation 115

4.3. Result and discussion 116

4.3.1. Challenges in delineating dopants characterized by minimal differences in atomic number 116

4.3.2. Quantitative analysis of dopants in Nb, Ni doped 1L-WS₂ 118

4.3.3. Quantitative strain analysis between transition metals. 120

4.4. Conclusion 124

References 125

논문요약 128

Figure 1.1. Examples of defect characterization of 1L-TMDs using Raman spectroscopy and STM. 17

Figure 1.2. Schematic illustration depicting the STEM imaging process. 21

Figure 1.3. Four types of imaging modes in STEM with its brief characteristics. 25

Figure 1.4. Dynamic scattering calculation : Multi-slice algorithm. 27

Figure 1.5. Defining reliable reference for precise vacancy quantification in 1L-TMDs using STEM-HAADF image and simulation image. 29

Figure 1.6. Accuracy of vacancy quantification. 30

Figure 1.7. Vacancy quantification in various 2D-TMDs. 31

Figure 1.8. Necessity of defining reliable FOV for vacancy quantification. Vacancy quantification results from low-FOV images showed much higher error... 32

Figure 1.9. Minimum reliable FOV for vacancy quantification. Based on various data sets with different S vacancy and FOV, we could say the minimum reliable... 33

Figure 1.10. Maximum reliable FOV for vacancy quantification. Since it gets hard to distinguish VS sites in 20 × 20 nm² FOV, we could set the... 35

Figure 1.11. Defect characterization in 1L-TMDs using STEM-HAADF image and vacancy calculation method. 36

Figure 2.1. PL characterization. (a) Optic images of three different grains of 1L-MoS₂ deposited on TEM grid. The PL intensity images are superimposed... 47

Figure 2.2. Raman characterization and determination of lattice parameters using DFT. (a) Raman spectra of both TFSI-treated (top panel) and pristine... 49

Figure 2.3. Quantifying sulfur and vacancy concentrations in the pristine 1L-MoS₂. (a) Five different HAADF-STEM images from pristine 1L-MoS₂ (top... 52

Figure 2.4. Quantifying sulfur and vacancy concentrations in the TFSI-treated 1L-MoS₂ (a) Five different HAADF-STEM images from TFSI-treated 1L-... 55

Figure 2.5. Influence of statistical noise when quantifying S-vacancy concentration in pristine 1L-MoS₂. (a, b) Experimental and simulated HAADF-... 57

Figure 2.6. Chemical composition analysis using STEM-EDX (a, b) EDX maps of pristine and TFSI-treated sample, respectively. F and N elements are... 58

Figure 2.7. Healing effect on S- and W-vacancies in hexagonal 1L-WS₂ by TFSI treatment. (a) HAADF-STEM images and corresponding vacancy maps... 60

Figure 2.8. Observation of structural damage induced by the electron beam in TFSI-treated 1L-MoS₂. (a) A series of STEM-HAADF images. (b) A graph... 62

Figure 2.9. Schematic model of S-vacancy healing mechanism by TFSI treatment. 64

Figure 3.1. Schematic illustration of electron doping process by Li treatment. 80

Figure 3.2. Evolution of nanodomains in 1T′-ReS₂ induced by Li treatment. (a, b) Comparative analysis of atomic structures between pristine 1T′-ReS₂ and... 82

Figure 3.3. Quantitative analysis of lattice distortion in Li-treated ReS₂. (a-c) Atomic-scale STEM-HAADF images of pristine, 6 h treated, and 9 h treated... 87

Figure 3.4. Quantitative measurements of lattice distortion which were conducted from three discernible domains exhibiting different DM chain arrays... 92

Figure 3.5. Observation of structural damage induced by the elect0ron beam in 1T′-ReS₂. A sequence of HAADF-STEM images was captured under 80 kV... 93

Figure 3.6. Chemical analysis using STEM-EDX. (a) A sequence of elemental maps of Re L (8.651 keV) and S K (2.307 keV) peaks from the ReS₂ samples... 95

Figure 3.7. Lattice symmetry change and phonon renormalization along Li treatment. (a) Raman spectra of 1L-ReS₂ along Li treatment time. (b) Phonon... 96

Figure 3.8. Determination of the layer thickness of exfoliated ReS₂ crystals (a) optic image showing various layer thickness. (b) Comparison of the peak... 98

Figure 3.9. XPS analysis of both pristine and treated ReS₂ crystal. (a) Optic image displaying the distinct layers of mechanically exfoliated ReS₂. Scale bar... 99

Figure 3.10. Polarization-resolved optical observations. (a) Optical reflection images before Li treatment.(b) Optical reflection images after Li treatment.... 101

Figure 3.11. PL quenching and DFT calculation of the electronic band structure. (a) PL spectra of pristine and treated sample.(b) Calculated electronic band... 103

Figure 3.12. Observation of alteration of the electrical transport after Li treatment. (a) Ids-Vgs characteristics of pristine sample. (b) Ids-Vgs...[이미지참조] 105

Figure 4.1. Schematic illustration of doping V and Co in 1L-WS₂ and quantitative analysis of dopant site in simulated HAADF image of V and Co doped 1L-WS₂. 117

Figure 4.2. Schematic illustration of doping V and Co in 1L-WS₂ and quantitative analysis of dopant site in simulated HAADF image of V and Co doped 1L-WS₂. 118

Figure 4.3. STEM-HAADF image and corresponding dopant distribution map in Ni, Nb doped 1L-WS₂. Nb and Ni dopants can be easily noticed because they... 119

Figure 4.4. Distance distribution map in a form of histogram. Only two peak (W and Nb/Ni) could be found. 120

Figure 4.5. Defining the direction of atomic distance between transition metal atoms. They were named as a₁, a₂, and a₃, respectively. 121

Figure 4.6. Strain map between transition metal atoms and corresponding histograms which shows the distribution of atomic distance in all three... 122

Figure 4.7. Strain map between W and Nb/Ni with corresponding histograms which shows the distribution of atomic distance in all three directions. 123

Figure 4.8. Quantitative dopant mapping result in Nb, Ni doped 1L-WS₂. 124